As a transmission of a vehicle, a mechanical automatic transmission in which a torque converter is not used is known. In the mechanical automatic transmission, operation (select and shift) of a gearbox and disconnection and connection of a clutch in a manual transmission are actuated by an actuator to enable automatic gear shifting that does not require a torque converter. In the transmission, for example, as described in Patent Literature 1, a gear group fixedly provided in first and second sub-input shafts coaxial with a main input shaft and a gear group supported by first and second output shafts and connected to the first and second output shafts by first and second meshing synchronization devices are meshed with each other, and first sub-shaft first and second gears supported by a first sub-shaft and connectable to each other by a third meshing coupling mechanism are respectively meshed with two gears of the first output shaft to establish at least forward six positions of gear positions and reduce an axial-direction dimension of the transmission.